Fascination with the Holy Grail has lasted for centuries, and now the Bletchley Park code-breakers have joined the hunt. But what is it that's made the grail the definition of something humans are always searching for but never actually finding?

Could an obscure inscription on a 250-year-old monument in a Staffordshire garden point the way to the Holy Grail - the jewelled chalice reportedly used by Jesus and his disciples at the Last Supper?

That is one theory entertained by Richard Kemp, the general manager of Lord Lichfield's Shugborough estate in Staffs.

Kemp has called in world-renowned code-breakers to try to decipher a cryptic message carved into the Shepherd's Monument on the Lichfield estate.

The monument, built around 1748, features an image of one of Nicholas Poussin's paintings, and beneath it the letters "D.O.U.O.S.V.A.V.V.M."

It has long been rumoured that these letters - which have baffled some of the greatest minds over the past 250 years, including Charles Darwin's and Josiah Wedgwood's - provide clues to the whereabouts of Christ's elusive cup.

Spot of bother

Poussin was said by some to have been a Grand Master of the Knights Templar, named after the order that captured Jerusalem during the Crusades and who were known as the "keepers of the Holy Grail".

Oliver and Sheila Lawn, with the mysterious inscription
Yet Oliver and Sheila Lawn, a couple in their 80s who were based at the code-breaking Bletchley Park in Buckinghamshire during World War II, have had a spot of bother with the Shepherd's Monument.

Mr Lawn said yesterday that deciphering the letters was "much more difficult" than cracking the Enigma code in WWII. He thinks it's a message from an obscure Christian sect, declaring their belief that Jesus was an Earthly prophet, not a divinity - while his wife Sheila thinks it could be a coded tribute from a widowed earl to his wife.

So yet another trail to the Grail seems to have run dry. What is it about the Holy Grail that so excites the popular imagination? And why are so many willing to believe that such an item exists, when there is a dearth of evidence?

Renewed interest

The Holy Grail is believed by some to have been the chalice used at the Last Supper, by others to have been a cup used by Joseph of Arimathea to catch the blood of the crucified Christ, and by others still to have been both. Some claim that Joseph may have brought the cup to Britain in the first century CE.

Stories about the Grail have been told for centuries. There has been a renewed interest since the publication of The Holy Blood and the Holy Grail in 1982, which claims, in a nutshell, that Jesus survived the crucifixion and together with Mary Magdalene founded a bloodline in France, the Merovingians, who were protected by the Knights Templar and later by the Freemasons. (Perhaps unsurprisingly, that book has been denounced as mad conspiracy-mongering by some.)

The Holy Grail has even turned up in Hollywood. In Steven Spielberg's Indiana Jones and the Last Crusade, the eponymous hero both fights off the Nazis and finds the Grail.

Now Ron Howard, the Happy Days actor turned film director, is making a big-screen version of The Da Vinci Code, Dan Brown's novel about how clues in Da Vinci paintings could lead to the discovery of a religious mystery, including the Grail, and shake the foundations of Christianity. Brown's novel has become a publishing phenomenon over the past two years, feted and hated in equal measure.

Purely legendary

According to experts, this is precisely where the Grail belongs - in fiction and films. Eric Eve is a tutor in theology and a New Testament scholar at Oxford University. He says he is unaware of any evidence for the existence of a Holy Grail.

Does Leonardo's Last Supper contain clues?
"In the version of the legend I know, the Grail is meant to be the chalice Jesus used at the Last Supper, subsequently brought to England by Joseph of Arimathea. But there is no 1st Century evidence about what happened either to the chalice or to Joseph - assuming he's even an historical character.

"The probability that the cup found its ways to Joseph and that he travelled with it to Britain is as near as nil as makes no difference. I would say it is purely legendary."

Richard Barber, author of The Holy Grail: The History of a Legend, published by Penguin next month, says the Grail legend came into being more than a thousand years after Christ's death.

"It is pure literature. It was imagined by a French writer, Chretien de Troyes, at the end of the 12th Century, in the romance of Perceval. His vision is at the root of all the Grail stories."

Conspiracy theories

Barber believes that 20th Century fascination with the Grail stems from "the revival of interest in medieval literature in the 19th Century, when Tennyson, Wagner and the Pre-Raphaelite artists were all enthusiasts for the Grail legends" - and that our fascination today has been boosted by the contemporary penchant for conspiracy theories and cover-ups.

"The Grail - because it is mysterious and has always belonged in the realms of the imagination - is a marvellous focus for the new genre of 'imagined history', the idea that all history as taught and recorded is a vast cover-up. Once this kind of idea becomes current, particularly with the internet, it acquires a life of its own - regardless of whether it has any basis in reality.

Even some of those who have written of the Grail as having some "basis in reality" admit that it is difficult to say what the Grail is, never mind where it is.

Erling Haagensen is co-author (with Henry Lincoln) of The Templars' Secret Island: The Knights, The Priest and The Treasure, which claims that "something" is hidden on the tiny island of Bornholm in the Baltic Sea.

"I do not know what the Holy Grail is," says Haagensen. "Something very important and with strong connections to the Holy Grail is hidden on the island of Bornholm. The Ark of the Covenant might theoretically be hidden there.

"But there is something even more important, which always followed the Ark of the Covenant, and which we can now prove is found at Bornholm. This will be revealed in our coming book," he adds, mysteriously.

Yet while some authors - and a host of conspiracy websites - believe that "something" will one day be found, even men of the cloth have little faith in the existence of the Holy Grail.

"It's all good fun but absolute nonsense", says Richard Holloway, former Bishop of Edinburgh. "The quest for the Holy Grail belongs with the quest for the ark Noah left on Mount Ararat or the fabled Ark of the Covenant Indiana Jones is always chasing. There ain't any objective truth in any of it - but of course it's a dream for publishers, who know the world is full of gullible people looking for miracles and they keep on promising that this time the miracle's going to come true.